Hi,

Worm gearboxes used to be standard issue for smaller, and not so small conveyor drives, because of price, compactness and runback resistance. Right angle shafts are built in and the motor sits closer to the drive. Worm gears give a higher reduction for less parts etc etc. Often the brake was the more cumbersome item. The casing is not so easy to split because the parting face is not amenable to direct overhead lifting access. Restricted ratios should be even less of an issue in this age of variable speed drives.

Environmental impact differences are negligible. A worm or helical gearbox uses far less mineral oil than a modern small motor car which burns oil as matter of course. It is easy to find a lube oil reclamation or reconditioning service at most doorsteps. It is essential that the full worm length is lubricated which means that for even lighter duty application a pump is required if the worm shaft is not horizontal. ■

Hello,

The oil consumption data / record is part of maintenance work, if one has kept record of the same. Often such record will not show any segregation with respect to the quantity of oil used for different equipment or their items.

The maintenance engineer will be the right person to opine on this subject. I am not maintenance engineer and cannot opine on the basis of earlier observation.

However, the subject analysis and some thoughts are as under:

1) Helical gear box efficiency happens to be around 94 to 96%.

2) Worm gear box efficiency is somewhat tricky and can range from 70 to 85%, depending upon ratio, kW, etc.

3) The power loss due to above efficiency gets converted into heat. Suppose drive power is 50 kW. Then the energy converted into heat for helical gear box will be 0.05 x 50 = 2.5 kW.

In case of worm gear box, the energy converted into heat will be 0.2 x 50 = 10 kW.

4) The kW loss as above will raise the temperature of oil and casing. The temperature will get stabilised according to: Heat produced = Heat loss.

5) The temperature rise will be causing ‘silent’ evaporation of oil. The worm gear oil will have more temperature and more vapour. So it will tend to result into more loss of the oil.

6) The oil and vapour loss would be at seals. The worm gear has 2 shafts and 4 sealing points. The corresponding ratio of helical gear box is likely to have 3 shafts and 6 sealing points. So the seals will tend to result in more oil loss for helical gear box.

Sr. nos. 5 and 6 will tend to compensate each other (not equally).

7) Then the other issue is manufacturer’s recommended number of oil changes in a year for worm gear and alternatively for helical gear. This multiplied by quantity of oil required for each change will give the quantity of oil used in a year for worm gear and helical gear.

The final effect will be due to combined influence of sr. nos. 5, 6 and 7.

It is to be noted that such comparison can be only on the basis of oil consumed per kW rating of gear box. This is due to reason that helical gear boxes are likely to be for higher kW (bigger size) compared to lower kW of worm gear (smaller size).

8) Above describes understanding of the subject. The factual comparison can only be by maintenance record (if some plant has manpower to attend such subject carefully).

Ishwar G. Mulani

Author of Book: ‘Engineering Science And Application Design For Belt Conveyors’. Conveyor design basis ISO (thereby book is helpful to design conveyors as per national standards of most of the countries across world). New print Nov., 2012.

Author of Book: ‘Belt Feeder Design And Hopper Bin Silo’

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India. Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■